The present invention relates to a method for making a lithographic printing master by exposing an imaging material to heat or light and processing the exposed material with a single-fluid ink.
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its is surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate setter. In the field of such computer-to-plate methods the following improvements are being studied presently:
(i) On-press imaging. A special type of a computer-to-plate process involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. Typical plate materials used in computer-to-press methods are based on ablation. A problem associated with ablative plates is the generator of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require wet processing with chemicals which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like support and one or more Functional coatings, computer-to-press methods have been described, e.g. in GB1546532, wherein a composition, which is capable to form a lithographic surface upon image-wise exposure and optional processing, is provided directly on the surface or a plate cylinder of the press. EP-A 101 266 describes the coating of a hydrophobic layer directly on the hydrophilic surface of a plate cylinder. After removal of the non-printing areas by ablation, a master is obtained. However, ablation should be avoided in computer-to-press methods, as discussed above. U.S. Pat. No. 5,713,287 describes a computer-to-press method wherein a so-called switchable polymer such as tetrahydro-pyranyl methylmethacrylate is applied directly on the surface of a plate cylinder ache switchable polymer is converted from a first water-sensitive property to an opposite water-sensitive property by image-wise exposure. The latter method requires a curing step and the polymers are quite expensive because they are thermally unstable and therefore difficult to synthesize.
(iii) Thermal imaging. Most of the computer-to-press methods referred to above use so-called thermal or heat-mode materials, i.e. plate precursors or on-press coatable compositions which comprise a compound that converts absorbed light into heat. The heat which is generated on image-wise exposure triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained.
(iv) The development of functional coatings which require no wet processing or may be processed with plain water, ink or fountain is another major trend in plate-making. Such materials are especially desired in computer-to-press methods so as to avoid damage or contamination of the optics and electronics of the integrated image-setter by contact with the processing liquids. WO 90002044, WO 91008108 and EP-A 580 394 disclose such plates, which are, however, all ablative plates having a multi-layer structure which makes them less suitable for on-press coating. U.S. Pat. No. 6,095,048 discloses the processing of an ablation-type material with a single-fluid ink.
A non-ablative plate which can be processed with fountain and ink is described in EP-B 770 494. The latter patent specification discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby converting the exposed areas into an hydrophobic phase which define the printing areas of the printing master. The press run car, be started immediately after exposure without any additional treatment because the layer is processed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is xe2x80x98hiddenxe2x80x99 to the user and accomplished during the first runs of the printing press.
A problem associated with the latter method is that the on-press processing is done by the steps of first supplying fountain to the plate and subsequently also ink, which can easily be carried out in printing presses wherein the ink and fountain rollers can be engaged independently from one another. However, it is more difficult to optimize on-press processing by the simultaneous application of fountain and ink, which is the only option in printing presses which are equipped with an integrated ink/fountain supply. Such presses supply a pre-mixed ink-fountain emulsion to the plate. Also driographic presses, which do not comprise a fountain supply, are unsuitable for on-press processing of materials which require both ink and fountain for their processing.
It is an object of the present invention to provide a method of making a lithographic printing master which simplifies on-press processing in driographic presses or in conventional wet-offset printing presses that comprise an integrated ink/fountain supply. This object is realized by the method of claim 1. It has been found that excellent results can be obtained by using a single-fluid ink for the on-press processing of an imaging material comprising a lithographic base and a non-ablative image-recording layer provided thereon, which is soluble in such a single-fluid ink or can be rendered soluble in the single-fluid ink by the exposure step.
Single-fluid ink is generally understood as an emulsion of an ink phase in a polar phase, or vice-versa, an emulsion of a polar phase in an ink phase. The polar phase is non-aqueous, meaning that the polar phase comprises at least 50%, more preferably at least 70% and even more preferably at least 90% of a non-aqueous, polar liquid. In a most preferred embodiment, the polar phase consists of an organic, polar liquid and comprises essentially no water. The polar liquid is preferably a polyol.
Single-fluid ink allows printing with a conventional, wet lithographic printing master without the application of a dampening liquid. The ink phase adsorbs onto the hydrophobic areas of the printing master and the polar phase wets the hydrophilic areas, thereby preventing adsorption of the ink component on the non-printing portions of the lithographic image.
The steps of exposing and processing may be carried out off-press, i.e. using an exposure apparatus (e.g. a plate-setter) that is not integrated in the printing press and which may be mechanically coupled to a processor. More preferably, the steps of exposing and/or processing are performed on-press, i.e. while the imaging material is mounted on a print cylinder of a rotary printing press, by means of an exposure apparatus that is integrated in the printing press. In the most preferred embodiment, both the exposure and the processing step are performed on-press, and then it is highly preferred to use the same single-fluid ink for the processing step as well as during the subsequent printing. In that embodiment, the steps of processing and printing are part of the same operation: after exposure, the printing process is started by feeding single-fluid ink to the material; after the first few revolutions of the is print cylinder (less than 20, more preferably less than 10), the imaging layer is completely processed and subsequently high-quality printed copies are obtained throughout the press run.
Further objects of the present invention will become apparent from the detailed description. Specific features for preferred embodiments of the invention are set out in the dependent claims.